(1) Field of the Invention
The invention relates to a delta modulation decoder for converting a digital signal into an analog signal, this digital signal being constituted by a sequence of pulses of a first kind, for example "1" pulses, and of pulses of a second kind, for example "0" pulses. Although a delta modulation decoder is used at both the transmitting end and at the receiving end of a transmission system for transmitting analog signals by means of delta modulation the present delta modulation decoder is particularly intended for use at the receiving end of such a transmission system.
(2) Description of the Prior Art
Particulaly advantageous embodiments of delta modulation decoders are disclosed in references 1 and 2 (chapter D).
In general, such decoders comprise:
an integrating network including a first integration capacitor as well as a first input and output terminal and a common second input and output terminal, PA1 means to which the digital signal is applied for producing in response to each of the pulses of the first kind a positive charge quantum and for producing a negative charge quantum in response to each of the pulses of the second kind, PA1 means for applying said charge quanta to said first integrating capacitor.
By applying a positive charge quantum the output voltage of the integrating network is increased by a given value. By applying a negative charge quantum the output voltage of the integrating network is reduced by the same value. One of the problems generally occurring in a transmitter and in a receiver of a delta modulation transmission system is that, in the transmitter as well as in the receiver, the magnitude of the positive charge quanta is never equal to the magnitude of the negative charge quanta. If, in addition, the difference between the magnitude of the positive and the magnitude of the negative charge quanta in the transmitter are different from the difference between the magnitude of the positive and negative charge quanta in the receiver, serious deviations from the desired linearity of the signal transmission are then produced.
To keep these linearity deviations of the signal transmission below a permissible limit, it is essential, both for the transmitters and for the receivers described in references 1 and 2, that the positive and negative charge quanta are exactly identical. A completely different solution to keep the linearity deviations of the signal transmission below a permissible limit, which results in a completely new set-up of a delta modulation decoder, is described in reference 3. The operation of this prior art decoder is based on the transport of charge from a first capacitor to the integrating capacitor in the integrating network. In this prior art device the magnitude of the positive charge quantum is each time determined by the output voltage of the integrating network. To this end, this output voltage is applied to a compensation circuit which ensures that the magnitude of the voltage across the first capacitor is made approximately equal to the magnitude of the output voltage of the integrating network. In contradistinction to the arrangements described in references 1 and 2, it is ensured in this prior art delta modulation decoder that the ratio of the magnitude of positive and negative charge quanta is equal to the ratio of the magnitude of positive and negative charge quanta used in the transmitter (delta-modulation coder).